PID stands for Proportional-Integral-Derivative, it is a closed loop sensor feedback system. It provides smooth and responsive reaction to sensor input changes that allow for robots that can follow, balance, and avoid among other reactions. The process starts with a proportional measurement of error from a set goal. The system attempts to correct for the error by reacting in the opposite direction, proportionally. This alone does not handle changes over time, a change in sensors that changes at a constant rate forces the P section to never reach its set goal. The integral portion of PID Loops allows the system to catch up to the set goal before maintaining a proportional value. The last part of a PID system is the derivative, the rate of change at any point in time. This calculation provides rapid responses to rapid changes. The wikipeida article on this topic is a little heavy but contains images that make understanding PID Loops easier.